Starting about 30 years ago, the steel industry has been gradually adopting the so-called Direct Reduction processes (DR), first only in plants of relatively small capacity, less than 500,000 tons per year, and lately at increasingly larger capacity, from 3 to 4 million tons per year. See for example U.S. Pat. Nos. 3,765,872; 4,046,577 and 4,150,972. DR plants provide an alternate route to obtain metallic iron from iron ores, mainly iron oxides, wherein iron-bearing particles are treated in the solid state with a reducing gas at high temperature in a reduction reactor, thus obtaining an intermediate product known as sponge iron or Direct Reduced Iron (DRI).
DRI is a solid material resulting from the reaction of said iron ores with a reducing gas which is mainly composed of hydrogen and carbon monoxide. The reducing gas is normally produced by reformation of natural gas with steam in a catalytic reformer. There have been developments however which permit running a DR process without said catalytic reformer by feeding natural gas directly to the reduction reactor. See for example U.S. Pat. No. 4,528,030.
Feedstock materials for DR processes comprise high grade iron ores, e.g. having a total iron content higher than 60%, whether in lump form with particle sizes between 0.37 and 1.5 inches, or pellets, of a particle size between 0.37 and 0.75 inches, made by grinding iron ores and magnetically concentrating the iron content thereof and finally subjecting said "green" pellets to an induration process at temperatures above 1200.degree. C. It has been also a common practice to feed mixtures of lumps and pellets to DR reactors thus overcoming certain operational problems of DR plants.
The essence of the DR processes is to be able to bring into uniform contact with a mass of solid iron-bearing particles (lumps or pellets) a stream of a reducing gas, at a temperature above 850.degree. C., to effect the reaction of iron oxide content to produce metallic iron in the solid particles and water and carbon dioxide as gaseous by-products.
These reduction reactions take place in a reactor, usually referred to as a shaft furnace, which may be of the moving bed type, fixed bed type, fluidized bed type, or rotary kiln type. The preferred reactor, employed nowadays in the steel industry is the moving bed type, followed by the fixed bed. Fluidized bed reactors have not yet been developed to a satisfactory degree of reliability and rotary kilns present scale-up problems for large capacities.
One of the principal problems affecting the productivity of DR plants of the moving bed and fixed bed type is the so called clustering of iron-bearing particles in the reduction reactor. The hot particles have a tendency to stick together in agglomerations (which adversely affect evenness of processing and subsequent handling). In a moving bed reactor, the iron-bearing particles are introduced thereto at its upper part, the bed of particles flows downwardly by action of gravity and are discharged at the lower part of said reactor through a solids flow regulating device which may be a vibratory feeder, a rotary or "star" valve, a "screw" feeder, or the like.
Ever since the initial uses of Direct Reduction processes in the 1950's and before, clustering problems have been known to occur with many iron ores and have been identified with the iron ore source. Certain ores have been found to have a higher sticking tendency than others, and the strength of the clusters formed increase with the temperature at which said ores are processed. Methods have been devised for predicting whether a given ore will present operational problems during reduction at a predetermined temperature. Accordingly, process operators even now maintain the process temperature in the reactor below a certain upper temperature limit characteristic of each iron ore. This measure however affects the productivity of the reactor (it being well known that the kinetics of the chemical reactions are exponentially dependent on the temperature).
Clusters cause many problems in the operation of the reduction reactor, for example gas channeling, obstructions (especially in zones of small cross-sectional area), instability of the bed, and sometimes the flow of solid particles through the reactor stops altogether with consequent losses in productivity and damage to equipment.
Clustering becomes aggravated in large reactors due to the high "ferrostatic" pressures (i.e., high mechanical pressures exerted on the iron-bearing particles) from the weight of the burden of particles above, due to the time of interparticle contact, and due to the geometric characteristics of the moving or fixed bed reduction reactors.
There has long been a desire and need to find a way of increasing the temperature in DR reactors, and consequently their productivity, and at the same time to inhibit the sticking tendency of iron-bearing particles.
Without intending to link a particular theory or explanation of the phenomena involved in the reduction process with the present invention, applicants believe that during the reduction process, metallic iron is formed at the surface of the iron-bearing particles which are in close contact with other particles surrounding them, and that due to the high temperature, strong interparticle bonds are developed at the points of contact resulting in clusters or agglomerates. Therefore, it is thought that coating the iron-bearing particles with some material that does not have deleterious effects on the reduction reactions and yet inhibits the formation of interparticle bonds, will permit a higher processing temperature in the reactor.
They also believe that by coating the particles with some such material, this will act as a "lubricant" between said particles, modifying the interparticle friction thus permitting a smoother flow of said particles through the reactor.
There have been several proposals addressed to solve the clustering problems. For example, U.S. Pat. Nos. 2,862,808; 4,118,017; 4,374,585 and 4,449,671 show mechanical means mounted at several places in the reactor to destroy clusters that may form. This solution however presents a number of drawbacks because movement of such steel racks or probes in the particles bed break and grind such particles producing dust and fines which in turn can cause gas channeling, and fines losses at the discharge. Furthermore, this type of mechanism needs special seals in the wall of the reactor and also require more capital and maintenance costs.
Another proposal to solve this problem is disclosed in German Laid-open Patent Application No. 2,061,346 comprising forming a coating of a ceramic powder on iron ore pellets. This ceramic powder can be limestone, dolomite, talc (basic magnesium silicate), lime or cement. This patent application however does not include or suggest any specific method or apparatus to apply said coating on the iron ore pellets. The significant problem of finding a way of handling a suspension of cement in large industrial quantities remains.
A later similar proposal is disclosed by K. Narita, D. Kaneko and Y. Kimura, "Study on Clustering and its Prevention in the Shaft Furnace for Direct Reduction Process", Trans. ISIJ, Vol. 20, pp. 228-235, 1980. This paper teaches to coat the surface of high grade pellets with substances such as CaO, MgO and carbon, to prevent clustering in a direct reduction reactor. This paper however also does not teach or suggest any method or apparatus to carry out such coating of pellets.
Applicants' earlier co-workers have disclosed in U.S. Pat. No. 4,388,116 the passivation of sponge iron pellets by coating with a mixture of Portland cement and powdered iron oxide. The patent's coating method entails the pre-wetting of the sponge iron pellets, followed by dusting with the cement/iron oxide dry fines, and thereafter requiring drying of the wet adherent coating in an inert atmosphere. It will be appreciated that the purpose as well as the process of this patent is different from the other references here discussed. The purpose of such other references is to prevent clustering occurring due to an agglomeration of hot iron ore pellets during the reduction process. However, in the passivation patent, it is the already-reduced sponge iron pellets, not iron ore pellets, which are treated. In passivation, the purpose is to prevent oxygen gas from passing into the reduced pellets and reoxidizing such sponge iron. In contrast, one significant aspect of the present invention concerns providing a porous coating on the iron ore pellets to eliminate clustering and yet permit free passage of the reducing gases into the interior of the pellets for completing the necessary reduction to sponge iron.
A further proposal for coating pellets with cement powder to avoid clustering is described in EPO patent No. 0 207 779. This patent publication suggests two methods and apparatus to carry out a coating process with cement. One is by immersing the pellets and the other is by spraying the pellets. Immersion of pellets is carried out by dropping the pellets from a bin into a tank holding a suspension of cement in water and withdrawing the pellets from the tank with a belt conveyor. The wet pellets then pass through a rotating drum-type drier having a screen to separate the fines and the excess of powdered cement. This method of applying the cement coating has the disadvantage of requiring a dryer, because the pellets are soaked with water, and therefore is costly in capital investment and operation. This patent does not teach or suggest how to avoid settling of cement powder in the tank, which is one of the main problems to deal with in practicing this method. The other disclosed method is to spray the cement suspension over the pellets. To this end, a cement suspension is prepared in a tank and then is pumped to a spraying nozzle which sprays the cement suspension over the pellets travelling in a belt conveyor. This spraying system has a number of drawbacks, for example, applicants were unable to find any pump which operates for a practical length of time without such pump becoming plugged and stuck by the cement settling and accumulating at all points in the pump and piping. Several types of pumps were tested and not one was found to function satisfactorily.
It is therefore an object of the invention to provide a non-clogging reliable method and apparatus for coating with cement iron-bearing particles with minimized wetting of the particles, which preferably are iron-ore particles to be processed in a Direct Reduction reactor so as to increase the productivity of direct reduction plants.